Effects of Aluminum Solids on the under Deposit Corrosion of Copper in Synthetic Potable Water: The Arguments for and against a Semi-Permeable Membrane

Abstract

Aluminum solids promote under deposit pitting of UNS C11000 copper. The possible roles that aluminum solids (i.e., Al(OH)3) play to promote copper pitting were examined by various characterization methods followed by diagnostic chemical/electrochemical methods in Edwards synthetic drinking water (ESDW). Cathodic polarization scans indicated that aluminum solids did not affect the intrinsic cathodic half-cell kinetics on an isolated copper electrode. Cyclic potentiodynamic polarization (CPP) experiments showed that the pitting potential (EPit) of copper was decreased by the presence of gelatinous aluminum solids deposited on the surface. Multi-electrode arrays (MEAs) with various surface deposition treatments were tested in ESDW under both driven and natural conditions. Lower pitting potentials were found under driven conditions on copper electrodes containing aluminum deposits. Pitting events also occurred on copper electrodes containing aluminum deposits with the presence of 5 ppm Cl2 under freely corroding conditions. MEAs were also tested with an inert Delrin crevice former. In this case, a lower E Pit and greater number of pitting events were found outside the crevice former. The drop in EPit was not as severe as in the case of deposited Al(OH)3, pointing to effects besides geometrically occluded sites, produced by inert crevice formers. Preferential sorption/transport of selected anions (OH-, HCO3-, SO 42-, and Cl-) through gelatinous Al(OH) 3 was studied. Plausible explanations for the detrimental effects of Al(OH)3 were given. © 2013 The Electrochemical Society. All rights reserved.